Kellgren was an early user of "phasing," a studio technique which simulates the sound of a jet engine.[3] He is also credited with pioneering other "psychedelic" sound effects, such as "flanging," a sound which is sometimes confused with Automatic double tracking (ADT).

Kellgren began his recording career at the Dick Charles demo recording studios in 1964 in the legendary Brill Building in Manhattan. Studios at that time had no décor to speak of. They "were sterile, utilitarian places. Engineers wore jackets and ties (or even lab coats!), and musicians performed under fluorescent lights and acoustical tile ceilings while seated on folding chairs. Amenities - if any - consisted of bad coffee and a few ashtrays. Record Plant broke that mold in a style that is now the stuff of legend."[4] And when Kellgren and his partner Chris Stone opened the Record Plant, it was Kellgren's concept to bring color, artistic design, hotel-like comforts and services to the world of recording studios along with state-of-the-art technology and acoustical design.

Gary was an institution. He single handedly was responsible for changing studios from what they were - fluorescent lights, white walls and hardwood floors – to the living rooms that they are today. His feeling, more than anyone else's, was that a studio should be a comfortable place to record. He was the one who first thought of the diversions, like the Jacuzzi he built in 1969. In those days that was unheard of. The only reason he built it was that I wouldn't agree to an Olympic size swimming pool in the parking lot. The Jacuzzi was Gary's compromise - and quite a surprise to me![5]

”

"There are tales told by the campfire, where rock mythology is discussed, that say the atmosphere created in that first studio and the ones to follow was so close to a good home and a fine hotel that songs were written to immortalize that special state of mind... It is safe to surmise that no other 'facility' has ever lent so much of a creative edge to the works of art at hand."[6]

In 1967, Kellgren and Stone built the first of the three Record Plant Recording Studios in Manhattan. Kellgren was the creative and recording side, Stone the business side, and their third parntner, Ancky Johnson, Revlon heiress, provided the financial backing. Kellgren had already been working with many of the top artists of the day, like Hendrix, so when he opened the Record Plant, they followed. Mitch Mitchell: "We went there [the Record Plant NY] because Gary Kellgren, who we'd worked with at Mayfair, had raised the money with a partner and managed to start the Record Plant."[7] The NY Record Plant was booked solid for three months in advance at the time of the opening.

The very first album to come out of there was Electric Ladyland by Jimi Hendrix. Stone later said in an interview that "Gary, having worked all night, would very often be out cold on the couch, with Jimi still in the studio glaring through the haze at the speakers. They would sometimes go three and four days without stopping. Out of those sessions came 'Electric Ladyland', the Record Plant's first record. Other people have taken credit for the record, but about ninety percent of it was done in Studio A in New York with Gary and Jimi."[5] One of the last to record there being John Lennon, who left the studio to go home the night he was murdered.

Almost two years after the launch of the New York Studio, Kellgren and Stone opened up the Los Angeles Record Plant. The first big mix session was for the Woodstock soundtrack. It was also where John Lennon and Paul McCartney played together for the last time during an all night jam session in 1974.

On July 20, 1977, Kellgren and his girlfriend/secretary Kristianne Gaines were found dead in the swimming pool at his residence in Hollywood. A business associate of Kellgren's was in the house at the time; he called police and reported that Kellgren had recently been in surgery, and that he had been swimming in the deep end of the pool. Gaines, 34, a resident of Los Angeles, was last seen alive sitting on a raft in the pool; she could not swim.[8] The incident was called "a double accidental drowning" by the police.[9] Two days later, an investigator told reporters that there was no indication of "foul play".[8] Guitarist Ronnie Wood wrote that Kellgren probably died from electric shock while trying to fix some underwater speakers in his pool, and that Gaines drowned trying to save him.[10]

Kellgren was survived by his wife Marta, their two children Devon and Mark, his sister Aleda, and his mother Crystal.